This invention relates to an optical glass of a SiO--B.sub.2 O.sub.3 --(TiO.sub.2 +ZrO.sub.2)--SrO--BaO--R'.sub.2 O (where R' represents an alkali metal element) system which is free of a toxic PbO ingredient, has an excellent property against devitrification, excellent light transmissivity and chemical durability and has optical constants of a refractive index (Nd) of 1.57-1.75 and Abbe number (.nu.d) of 30-55.
As a glass having the above mentioned optical constants, there is known a silicate glass containing PbO. For coping with environmental problems arising in the process of manufacturing a glass, however, various glasses which do not contain the toxic PbO ingredient but instead contain a TiO.sub.2 or ZrO.sub.2 ingredient are proposed. For example, Japanese Laid-open Patent Application No. 53-16717 discloses a SiO.sub.2 --TiO.sub.2 --ZrO.sub.2 --CaO--BaO--R'.sub.2 O system glass and Japanese Laid-open Patent Application No. 53-92816 discloses a SiO.sub.2 --TiO.sub.2 --ZrO.sub.2 --(ZnO+MgO+CaO+SrO)--BaO--Li.sub.2 O system glass. Also, Japanese Laid-open Patent Application No. 53-90312 discloses a SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --ZrO.sub.2 --ZnO--CaO--BaO system glass. These glasses, however, are insufficient in stability against devitrification, light transmissivity and chemical durability.
It is, therefore, an object of the invention to provide an optical glass having the above described optical constants which has overcome the above described drawbacks of the prior art optical glasses.